It is well-known that power generating units, such as wind power plants, can be operated so as to control the fundamental frequency of the power supply grid operatively connected to the power generating units.
In order to control the fundamental frequency of a power supply grid in an optimal manner information about the steady-state and the dynamic properties of the power supply grid is required.
A method for varying the active power provided by a wind energy installation is suggested in EP 1 914 419. The method suggested in EP 1 914 419 involves that the amount of injected active power from the wind energy installation is a function of the actual grid frequency. As disclosed in EP 1 914 419 the amount of active power, P, varies with the grid frequency, f, in the following manner:
  P  =                    K        1            ⁡              (                  f          -                      f            N                          )              +                  K        2            ⁢                        ⅆ          f                          ⅆ          t                    where f is the actual grid frequency, fN is the nominal grid frequency, and K1 and K2 are constants. The constants K1 and K2 are typically provided by the power distributor. Thus, the constants K1 and K2 are completely static and their values do not account for changes occurring in the power supply grid. It is an object of embodiments of the invention to provide a method and a control system applying actual and updated information about the stiffness and the inertia of a power supply grid in order to control the fundamental frequency of said power supply grid in an optimal manner.